Deglacial Lake Agassiz Meltwater Oxygen Isotope Geochemistry From Linked Ba-δ18O Analyses on Arctic N. pachyderma Shells

Throughout the last deglaciation there is evidence for multiple large inputs of ice-sheet derived meltwater into the Arctic Ocean, including at ~11.5 ka; this has been hypothesized as related to the draining of glacial Lake Agassiz. High-resolution ocean circulation models support the idea that a massive input of freshwater into the North Atlantic, possibly through the Arctic Ocean via the Mackenzie River, triggered a slowdown in Atlantic Meridional Overturning Circulation. It is suspected that this meltwater pulse initiated the northern hemisphere cold interval known as the Younger Dryas (YD). Quantifying the melt δ ¹⁸ O and potential time transgressive variations in Mackenzie River outflow during this event is an important first step in computing the volume of water entering the N. Atlantic from the Arctic during the YD.

Here we present geochemical results on a species of planktonic foraminifera, Neogloboquadrina pachyderma (Nps), from several intervals during a meltwater event recorded on the Mendeleev Ridge (AOS 94b-17, 2217m) that displays a large δ¹⁸O depletion between 11.2-12.4 cal kyr. We combine trace element measurements from laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with δ¹⁸O_calcite analyses on individual Nps shells to reconstruct the δ¹⁸O of the Lake Agassiz-derived Mackenzie River meltwater. We use LA-ICP-MS measurements of Ba/Ca in Nps shells to compute salinity utilizing a published salinity-Ba/Ca_shell relationship (Vetter et al., 2017). We combine the Ba/Ca calculated salinities from those same shells with computed δ¹⁸O_sw values to generate δ¹⁸O_sw vs. salinity regressions for each meltwater interval; y-intercepts of these regressions correspond to the δ¹⁸O composition of the freshwater (meltwater) endmembers. Results from box core AOS 94b-17 indicate that the δ¹⁸O_meltwater values of the Mackenzie River outflow slowly increased by ~10‰ between 12.4 cal kyr and the end of the YD event. Understanding the geochemical history of Lake Agassiz melt flowing into the Arctic Ocean through time provides new constraints on the volume and rates of Lake Agassiz discharge into the North Atlantic across the YD Event.

Vetter, L. et al., 2017. Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I. Quat. Sci. Rev. 178, 102-117.